I had the service engine light come on my truck recently. The code indicated that I had a computer power problem. I had let the battery run down to zilch when I left my GPS plugged in for about week. I reset the service engine warning, and all has been good since.....that puppy saved me about $80 in diagnostic cost at the local dealership.

I got the ScanGauge II originally to monitor my tranny temperature, but now I can monitor any number of items.

I know it cost a little more, BUT.
I have a Bully Dog GT Gas Tuner in my truck amd it will read and reset, as well as allow you to change engine program, shift points, speed limiters, adjust speedo for tire size among other things. I have been very pleased, I have the 4.8l in my truck and the power/mileage difference with the performance tune in is a tremendous difference. With my old camper I would get 9-10 mpg before I got the Tuner, after it I got 12+ on the same trips as before with basically te same weightti n the camper, but much more power left while towing in the mountains, unlike pre tuner!
I got mine at Advanced, but made them price match Oreily autoparts.